[css3-transforms] Why do we need a 'perspective' property?

We have a perspective() function already for transforms.  Why do we
also need a separate 'perspective' property?  If I want something like

<div style="height:100px;width:100px;perspective:1000px">
<div style="height:100px;width:100px;background:lime;
transform:rotatex(45deg)">
Some text</div></div>

why can't I just do

<div style="height:100px;width:100px;background:lime;
transform:perspective(1000px) rotatex(45deg)">
Some text</div>

instead?  Handling of 'perspective' seems to complicate 3D transforms
significantly, particularly the way it only applies to children and
not other descendants, and I don't understand the benefit.